Projecting non-visible light at a specimen and capturing the resulting fluorescence/photoluminescence emitted by a specimen can provide important information about the quantity, type, location and morphology of features on a specimen. Further, certain features of a specimen, such as the purity or structural imperfections of the specimen, among others, may only be observed using non-visible illumination. Specimens as understood by a person of ordinary skill in the art refer to an article of examination (e.g., a wafer or a biological slide) and features refer to observable characteristics of a specimen, including abnormalities and/or defects. Features can include but are not limited to: circuits, circuit board components, biological cells, tissue, defects (e.g., impurities, structural imperfections, irregularities, stacking faults, contaminants, crystallographic defects, scratches, dust, fingerprints).
Note, the term fluorescence (FL) as used herein includes photoluminescense, which is commonly associated with light emissions from semiconductor materials. Non-visible light refers to the region of the electroagnetic spectrum with a wavelength between 10 and 400 nanometers (nm) (i.e., the region between visible light and X-rays). In some embodiments, for example, light wavelengths in the range of 200 nm to 400 nm, 300 nm to 400 nm, and/or any other suitable wavelengths can be selected. Moreover, the light wavelength required to excite a specimen and cause fluoresencese by a specimen from the absorption of light or other electromagnetic radiation is not restricted to the wavelength range between 10 nm to 400 nm, but, in some embodiments, can be selected in a range above 400 nm to provide the desired excitation to a specimen, as explained herein. Coherent light refers to particles of light energy that have the same frequency and its waves are in phase with one another. In contrast, the particles of light energy of incoherent light do not have the same frequency and its waves are not in phase with one another.
While coherent light sources (e.g., lasers) are commonly used for specimen fluorescence, such light sources are not ideal for detecting large features or for use with certain types of specimens (e.g., patterned wafers). Incoherent light sources, on the other hand, are better suited for detecting a greater range of features (including large features and features on patterned wafers). Moreover, coherent light sources illuminate only a small portion of a field of view, whereas incoherent light illuminates the entire field of view, making it more suitable for creating specimen feature maps. Specimen feature maps classify features on a specimen and specify their location. Note, the term field of view as understood by a person of ordinary skill in the art refers to an area of examination that is captured at once by an image sensor. Further, a person of ordinary skill in the art will readily understand that the terms field of view and image are used interchangeably herein.
Accordingly, new fluorescence microscopy inspection mechanisms using incoherent illumination techniques are desirable to excite specific layers of a specimen or materials contained in a specimen to cause them to fluoresce and to automatically detect features of a specimen from the resulting fluorescence. Moreover, it is also desirable for the same mechanisms to inspect features of a specimen using illumination techniques that do not cause fluorescence.